Electroluminescence devices are used for display component such as thin film panels and cylindrical panels, surface luminous bodies such as large-area panels, and various other devices. In particular, recently, electroluminescence devices have begun to be widely used for oscillation device of optoelectronics such as electronic devices for laser beam, and so on.
The electroluminescence device is classified into an inorganic electroluminescence device in which an inorganic compound is used in the emissive layer and an organic electroluminescence device in which an organic compound is used in the emissive layer.
The inorganic electroluminescence device is a system in which an inorganic compound is sandwiched by insulating layers and which is driven by application of an alternating current voltage and is a genuine electroluminescence device in which a high-speed electron accelerated in the high electric field collides with and excites the luminescent center portion. The inorganic electroluminescence device is put into practical use in, for example, a green luminescent display.
The organic electroluminescence device has a structure in which a thin film containing an organic compound is interposed between an anode and a cathode and is a type in which electrons and holes are injected in this thin film, thereby undergoing luminescence by the energy of their recombination (also called “charge injection type”). The organic electroluminescence device can emit light of high luminance at a low direct current voltage of from several volts and several tens of volts and is expected to be applied to various luminescence devices, display devices, and the like.
As described previously, the inorganic electroluminescence device using an inorganic compound in the emissive layer is put into practical use in green luminescent displays, etc. However, in order to drive the inorganic electroluminescence device, an alternating current power supply and a high voltage are necessary, and places and ranges where the inorganic electroluminescence device can be used are limited.
In the light of the above, at present, studies regarding organic electroluminescence devices capable of emitting light with high luminance at a low direct current voltage are being intensively performed, and new studies for putting inorganic electroluminescence devices into practical use are not as numerous now. Accordingly, the results of useful studies (such as luminescence characteristics) regarding inorganic electroluminescence devices which have hitherto been accumulated are not thoroughly utilized.
On the other hand, as described previously, the organic electroluminescence device using an organic compound in the emissive layer can undergo luminescence with a high luminance at a direct current low voltage. However, in comparison with the inorganic electroluminescence device, the organic electroluminescence device involved such problems that it is inferior in deterioration characteristics (lifetime) of constituent materials thereof and that it is not durable when used over a long period of time.
Also, among organic electroluminescence devices, the most of those which are generally put into practical use have an emissive layer in which a guest dye (organic dyes and fluorescence dyes such as organometallic complexes) is doped in an organic compound layer host material. In order to obtain the luminescent color necessary for high efficiency by performing optimal dye doping, a relationship of physical and chemical properties between the host material and the guest dye (for example, necessity that an emission spectrum of the host material and an absorption spectrum of the guest dye overlap each other) is important.
In the light of the above, in order to obtain a necessary luminescent color, it is necessary to suitably combine the constituent materials of the host material and the guest material, so that for example, in the case of replacing the guest dye, it is necessary to replace the host material at the same time. Accordingly, for example, in the case of manufacturing a display using an electroluminescence device, the number of necessary host materials becomes high, leading to an increase of the costs for manufacturing the display.
Then, first of all, in order to make the most of the results of useful studies regarding inorganic electroluminescence devices which have hitherto been accumulated, the present inventors made extensive and intensive investigations on the basis of ideas regarding whether or not an inorganic compound to be used in the emissive layer of an inorganic electroluminescence device can be driven at a direct current voltage (low voltage) like an organic electroluminescence device and whether or not the deterioration characteristics that are a defect of an organic electroluminescence device can be solved.
As a result, it has been found that not only it is effective means for solving the foregoing problems to prepare an electroluminescence device by dispersing an inorganic compound in an organic compound layer provided between an anode and a cathode, but also it is possible to change the luminescent color of an electroluminescence device by dispersing an inorganic compound in an organic compound layer. Also, investigations were made regarding an inorganic compound to be dispersed. As a result, it has been found that it is possible to change a luminescent color by replacing an inorganic compound to be dispersed in an organic compound layer.
The invention has been completed based on these findings.
Then, an object of the invention is to provide an electroluminescence device capable of driving (causing light emission from) an inorganic compound at a direct current voltage (low voltage).
Also, another object of the invention is to provide an electroluminescence device capable of changing the luminescent color by dispersing an inorganic compound in an organic compound layer which is the emissive layer.
Further, other object of the invention is to provide an electroluminescence device in which the luminescent color changes by replacing an inorganic compound dispersed in an organic compound layer.
The other objects of the invention will become clear by the following description.